mpath: disable devmapper-multipath checking on non-linux

The configure script was breaking on MacOS X unless passed:

  --without-storage-mpath

This patch leverages Stefan Bergers earlier work for nwfilter,
so non-linux systems don't even attempt to build multipath.

configure.ac

@@ -1503,7 +1503,15 @@ if test "$with_storage_scsi" = "check"; then
 fi
 AM_CONDITIONAL([WITH_STORAGE_SCSI], [test "$with_storage_scsi" = "yes"])
 
-if test "$with_storage_mpath" = "check"; then
+with_linux=no
+case "$host" in
+  *-*-linux*)
+    with_linux=yes
+    ;;
+esac
+AM_CONDITIONAL([WITH_LINUX], [test "$with_linux" = "yes"])
+
+if test "$with_storage_mpath" = "check" && test "$with_linux" = "yes"; then
    with_storage_mpath=yes
 
    AC_DEFINE_UNQUOTED([WITH_STORAGE_MPATH], 1,
@@ -2022,14 +2030,6 @@ then
 fi
 AM_CONDITIONAL([WITH_NODE_DEVICES], [test "$with_nodedev" = "yes"])
 
-with_linux=no
-case "$host" in
-  *-*-linux*)
-    with_linux=yes
-    ;;
-esac
-AM_CONDITIONAL([WITH_LINUX], [test "$with_linux" = "yes"])
-
 
 with_nwfilter=yes
 if test "$with_libvirtd" = "no" || test "$with_linux" != "yes"; then